Roughening transition as a driving factor in the formation of self-ordered one-dimensional nanostructures

Based on the Monte Carlo kinetic method, we investigated the formation mechanisms of periodical modulations arising along the length of one-dimensional structures. The evolution of initially cylindrical nanowires/slabs at temperatures lower than their respective melting temperatures can result either in breakup into single nanoclusters or in the formation of stable states with pronounced modulations of cross section. Such configurations, observed in a number of experiments, are excited at wavelengths that are below the critical value for the development of classical Rayleigh instability. We show that the modulation excited in the subcritical mode corresponds to the appearance of roughening transition on the quasi-one-dimensional surface of nanowires/slabs. Since the arise of roughening transition is possible only on certain facets of metals with a given crystal structure, the short-wavelength modulations of one-dimensional systems, as shown in our work, can be realized (i) with the proper orientation of the nanowire/slab axis providing spontaneous appearance of roughening transition on its lateral surface, (ii) by the method of activating the surface diffusion of atoms by external impact (irradiation with an electron beam or contact with a cold plasma), which stimulates roughening transition without significant heating of the nanowire. The results obtained for the cases of BCC and FCC lattices can be used in the controlled synthesis of ordered one-dimensional structures for use in optoelectronics and in ultra-large-scale integrated circuits

Restructuring and breakup of nanowires with the diamond cubic crystal structure into nanoparticles

A kinetic Monte Carlo approach is applied to study physical mechanisms responsible for the breakup of nanowires with the diamond cubic crystal structure into a chain of nanoparticles discovered in preceding experiments on Silicon nanowires. We show that this process is based on the well-known mechanism of roughening transition, which specifically manifests itself in quasi-one-dimensional systems/nanowires with a pronounced anisotropy of the surface energy density. Depending on the temperature and orientation of the nanowire relative to its internal crystal structure, the wavelengths of substantial cross-sectional modulations exceed its initial radius by 4 to 18 times. For certain orientations, a straight nanowire at the initial stage of evolution forms a serpentine/helical structure. The scenarios of the stage of nanowire ruptures into single nanoclusters are also diverse: either each spindle-shaped region of the nanowire transforms into a separate drop (by long-wave surface perturbations), or the adjacent short-scale beads absorb each other due to the Ostwald ripening effect, which can be accompanied by the formation of long-lived many-body dumbbells. The discovered features of the dynamics of quasi-onedimensional systems expand our conceptions of the physical mechanisms involved in the breakup of nanowires (presented by Nichols and Mullins as a classical model for such instabilities) which could be useful in applications based on chains of ordered nanoparticles.Comment: 38 pages, 15 figures, 7 vide

Diversity of anisotropy effects in the breakup of metallic FCC nanowires into ordered nanodroplet chains

We have analyzed the expressed manifestation of the anisotropy of surface energy density in the dynamics of ultrathin nanowires, which break up into disjointed clusters when annealed below their melting temperature. The breakup process is studied for different temperatures and orientations of the nanowire axis relative to its internal crystal structure using the Monte Carlo kinetic method. We have also presented an approximate analytical model of the instability of nanowires. Generally, the interpretation of experimental results refers to the theoretical model developed by Nichols and Mullins, which is based on conceptions about the Rayleigh instability of liquid jets. In both cases, the theories - which do not take into account the anisotropy of surface energy density - predict the breakup of a nanowire/liquid jet with radius r into fragments with an average length {\Lambda}=9r. However, the observed value, {\Lambda}/r, often deviates from 9 either to lower values or to substantially greater ones (up to 24-30). Our results explain various observed features of the breakup and the significant variations in the values of its parameter {\Lambda}/r depending on experimental conditions. In particular, the ambiguous role of exchange by atoms of the surface of a nanowire with the surrounding layer of free atoms formed as a result of their rather intensive sublimation, which occurs in a number of cases, has been investigated. We have shown that this exchange can lead both to a decrease, and to a significant increase, in the parameter {\Lambda}/r. The obtained results could be potentially useful in applications such as the development of optical waveguides based on ordered nanoparticles chains